Infradian rhythms: biological cycles that take longer than twenty-four
hours. For example, women’s menstrual cycles occur about every twenty-eight
days.

Ultradian rhythms: biological cycles that occur more than once a day.
Sleep follows an ultradian rhythm of about ninety minutes as well as a
circadian rhythm. Alertness and hormone levels also follow ultradian
rhythms.

Biological rhythms usually synchronize with environmental events such as
changes in daylight. However, experiments have shown that many biological
rhythms continue to have the same cycle even without cues from the environment.
Such biological rhythms are endogenous, which means that they
originate from inside the body rather than depend on outside cues.

Biological Clocks

Endogenous rhythms exist because the body has biological clocks that keep
time. Biological clocks can be adjusted by environmental cues, such as changes
in temperature.

In humans, the suprachiasmatic nucleus (SCN) is the main biological clock that regulates circadian
rhythms of sleep. The SCN lies in the brain’s hypothalamus. When light
stimulates receptors in the retina of the eye, the receptors send signals to
the SCN. The SCN then sends signals to the nearby pineal gland,
which secretes melatonin, a hormone that regulates the sleep
cycle.

Jet Lag

Jet lag is the fatigue and disorientation air travelers feel
after a long flight. Although traveling itself drains energy, the time
change also contributes to fatigue. People experience jet lag when the
events in their environment are out of sync with their biological
clocks.